The present invention relates to a hydraulic clutch control system and method.
An automotive vehicle has a clutch mechanism arranged on a power transmission line to selectively connect and disconnect the transmission of power from a vehicle engine (such as internal combustion engine) or electric motor to wheels under vehicle drive/shift control. There are some types of clutch mechanisms including a friction clutch in which rotary elements are engaged to allow power transmission due to friction between the rotary elements. One example of friction clutch is known as a multiple disc clutch. The multiple disc clutch has first and second rotary elements and a clutch piston actuated by hydraulic pressure to press the first rotary elements against the second rotary elements and thereby engage these rotary elements such that the degree of engagement of the rotary elements can be controlled according to the hydraulic pressure on the clutch piston. The multiple disc clutch also has a return spring to bias the clutch piston apart from the rotary elements and leave some clearance between the clutch piston and the rotary element for complete disengagement of the rotary elements when the hydraulic clutch pressure becomes decreased to a certain level. In the case that the multiple disc clutch starts from such a complete disengagement state, the rotary elements cannot be engaged until the clutch piston comes into contact with the rotary element i.e. during a so-called idle stroke of the clutch piston. This results in slow clutch response.
Thus, it has become conventional to detect the amount of clearance between the clutch piston and the rotary element, calculate the hydraulic pressure to be exerted on the clutch piston to eliminate the detected clearance and complete the clutch piston idle stroke, and then, reflect the calculated value of the hydraulic clutch pressure in actual clutch control operation.
Japanese Patent No. 2595812 discloses a control device employing such a clutch control technique. The control device of Japanese Patent No. 2595812 is characteristically configured to limit the hydraulic clutch pressure to a first level (higher than the return spring tension and lower than the initial design clutch engagement force) during a predetermined time period, gradually increase the hydraulic clutch pressure to a second level at a given rate and, during the increase of hydraulic clutch pressure, judge the timing of completion of the clutch piston idle stroke (the timing of contact of the rotary elements) by sequential second-order differentiation of the actual hydraulic clutch pressure value.